Add support for disregarding the GitHub Actions env variables

[AArnott]

Set the IGNORE_GITHUB_REF environment variable to true when your GitHub workflow has moved HEAD such that the GITHUB_REF and GITHUB_SHA environment variables are stale.

Fixes dotnet/nbgv#50

[codecov]

Codecov Report

Merging #795 (d36ca82) into main (8524158) will decrease coverage by 0.02%.
The diff coverage is 0.00%.

@@            Coverage Diff             @@
##             main     #795      +/-   ##
==========================================
- Coverage   81.22%   81.20%   -0.03%     
==========================================
  Files          73       73              
  Lines        3830     3831       +1     
==========================================
  Hits         3111     3111              
- Misses        719      720       +1     

Impacted Files	Coverage Δ
....GitVersioning/CloudBuildServices/GitHubActions.cs	6.25% <0.00%> (-0.42%)	⬇️

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